1. Field of the Invention
This invention relates to catalysts and more particularly to alumina coated substrates, stable at high temperatures, and to methods of making the same.
2. Description of the Prior Art
Catalysts which are used to treat large volumes of gases having high space velocities, such as exhaust gases from internal combustion engines, preferably have large pores, good heat and mass transfer characteristics, low resistance to fluid flow, and low thermal mass. One example wherein such catalysts are useful is in the treatment of internal combustion engine exhaust gases, which are generally treated in order to reduce or eliminate certain constituents, such as carbon monoxide, oxides of nitrogen, hydrocarbons, lead halides and particulate matter, such as lead and carbon particles.
Alumina, coated and adhered to an extended substrate, then calcined and having added catalytic metal, if required, has been found to be a useful and effective catalyst for treating internal combustion engine exhaust gases. A thin layer of calcined alumina adhered to the surface of an extended substrate, such as a metal mesh, and used as a catalyst, has several advantages over other available structures, such as alumina coated ceramic monoliths or pelletized alumina. The packing density of alumina coated substrates can be very low and yet have a relatively large external alumina surface contacting the gas. Also, the low resistance to flow, the high rates of heat and mass transfer, and the low thermal mass of alumina coated substrates, favor the use of these structures, especially when the flowing gas is to be used to control the reaction temperature of the catalytic reaction on the alumina, or wherein particulate matter is to be filtered out by the porous alumina from the flowing gas.
Coassigned U.S. Pat. No. 3,231,520 discloses a preferred method of forming an adherent alumina film on a substrate. The alumina film can be used as a support for catalytic materials which are used to promote a variety of reactions including those useful in treating exhaust gases from internal combustion engines. The substrate on which the alumina film is formed can be a metal or nonmetal, and can have a variety of configurations. The adherent alumina film is disclosed as being formed by contacting the substrate with a sodium aluminate coating solution causing a hydrated adherent film of alumina trihydrate to be formed on the substrate. When the adherent alumina trihydrate film is dried and calcined, a hard film of porous alumina, sufficiently tenacious to withstand ordinary usage, forms on the substrate. Coassigned U.S. Pat. Nos. 3,227,659 and 3,410,651, disclose the impregnation of an alumina film, formed in a manner similar to that of U.S. Pat. No. 3,231,520, with phosphorus and chromium containing materials, respectively.
The alumina coated substrate is disclosed as being used without impregnation as a filter to remove lead compounds from internal combustion engine exhaust gases in coassigned British Pat. No. 1,271,710, and as a filter to remove carbon particles from diesel engine exhaust gases in coassigned U.S. Pat. No. 4,039,294.
Using the porous alumina coated substrate, (with or without added catalytic materials) to treat the exhaust gases of an internal combustion engine under normal operating conditions does not generally subject the alumina film and substrate to temperatures in excess of the calcining temperature initially used to form the porous alumina film. Under these normal conditions the alumina film remains intact and continues to perform satisfactorily. However, under certain abnormal operating conditions of an internal combustion engine, or because of the ignition of certain combustible materials which have built up on a limited portion of the alumina film, much higher temperatures (upwards of 1260.degree. C.) can be reached by portions of the alumina film. At such high temperatures, the surface area of the alumina film rapidly diminishes, dropping well below 10 square meters per gram of alumina, and further, the structural integrity of the alumina film rapidly degrades causing powdering and other structural failures. Both the decrease in surface area and the physical loss of alumina material results in a diminished surface area of alumina film and catalyst material available to treat the exhaust gases.
Previously, it has been disclosed that certain materials appear to stabilize calcined alumina at high temperatures, reducing the loss of surface area, and of structural integrity that would otherwise occur. U.S. Pat. No. 3,899,444 discloses several materials useful for this purpose including potassium, cesium, and barium compounds, and also rare earth metal oxides. U.S. Pat. No. 3,291,564 discloses some of these same materials. U.S. Pat. No. 3,172,866 discloses the use of boron oxide as a stabilizing material. A paper by James Roth et al. entitled "Control of Automotive Emmissions by Particulate Catalyst" published by Society of Automotive Engineers and presented at the International Autumotive Engineering Conference in Detroit, Mich. on Jan. 8-12, 1973, (paper No. 730,277) discloses several materials believed to stabilize calcined alumina at high temperatures resulting in a higher surface area than could be otherwise expected to occur. Also, certain materials are disclosed, which when added to the calcined alumina, produce an alumina with a smaller surface area at a lower temperature than untreated alumina.
In the prior art, the stabilizing material, when used, generally was just added to a slurry or mixture of calcined and powdered alumina, and a carrier. The mixture of stabilizing material, calcined alumina and carrier were then applied to a support or formed into a desired shape.
Attempting to incorporate stabilizing materials into the alumina film formed, for instance, by the method of U.S. Pat. No. 3,231,520, by the simple addition of the stabilizing materials to the basic sodium aluminate solution prior to or during contacting of the solution with a substrate, generally results in undesirable reactions between the stabilizing materials and the constituents of the sodium aluminate solution. The reactions can cause the precipitation of the stabilizing materials, preventing their inclusion in the alumina film. The addition of the stabilizing materials to the formed alumina trihydrate film prior to dehydration, results in the stabilizing materials being found only on the surface of the hydrated alumina film. When the alumina film is subsequently dehydrated, the stabilizing materials remain on the surface of the calcined alumina film, resulting in a lack of stabilization of a major portion of the calcined alumina film.